This invention relates to low-cost thermometers and to methods of making such thermometers. The thermometers of the present invention are particularly adaptable for use as low-cost clinical thermometers which may be disposed of after a single use.
As is well known, the typical clinical thermometer in present use is made from a glass tube in which mercury or other suitable expansible fluid for indicating temperature is contained in a bulb section and is free to expand in a capillary passage. The use of glass in the construction of prior art thermometers requires extended processing to assure dimensional stability of the capillary tube which influences the accuracy of the thermometer.
When glass thermometers are used, there is, of course, the ever present danger of breakage either during handling or use of the thermometer. This breakage is particularly dangerous with glass mercury thermometers because of the possible ingestion of glass fragments and of the toxicity of mercury. Moreover, the repeated use of glass thermometers by the same individual, or different individuals, requires periodic sterilization and raises the possibility of cross-contamination by virtue of inadequate or incomplete sterilization to the thermometer. This problem is particularly acute in hospitals, clinics and the like. Additionally, the external temperature scale graduations and numerals on the glass thermometer tend to fade and wear off during use and when repetitively subjected to aseptic solutions.
Another problem associated with thermometers is the retention of the expansible fluid, be it mercury or other fluids, at the indicated temperature of the patient. Especially with mercury thermometers, there is a strong tendency (due to the cohesiveness of the mercury) for the expansible fluid to return to the bulb section after the thermometer is removed from the patient. This problem has been solved in the prior art by the addition of a fluid constriction in the capillary tube, which breaks the column of mercury as it attempts to return to the bulb, thus preserving the reading. However, the constriction is difficult to fabricate and necessitates a shake-down of the mercury in the thermometer prior to subsequent use.
Because of the inherent problems associated with glass-mercury clinical thermometers, various types of nonbreakable, non-mercury thermometers have been proposed but as far as is known, none have received commercial acceptance. This failure has been due, at least in part, to deficiencies in the accuracy of the thermometers, and the inability to produce such thermometers at a sufficiently low-cost to permit them to be discarded after a single use.
As will be further described below, the thermometers of the present invention avoid the disadvantages of the prior art thermometers by using all plastic components which resist breakage, are readily sterilizable, and are internally marked to prevent fading of the graduations. These thermometers use a colored non-toxic expansible fluid which rises in the capillary passage to provide a readily visible indication of the temperature of the patient.